There are known systems for checking thickness dimensions of semiconductor slices, or wafers, in the course of machining on machine tools, more specifically grinding machines and polishing machines.
The known systems are of different kind and can include, for example, gauging heads with mechanical feelers that touch at least one surface of the wafer being machined. Systems of this kind, which may affect the checked piece and do not allow to fall below certain dimensional values, cannot be applied in the event there is needed, as frequently occurs, to accurately check the thickness of a semiconductor wafer having one side fixed to a film or a support element. Other known systems employ probes of a different kind, for example capacitive probes, inductive probes (eddy current probes or other) or ultrasonic probes. However, the limits of these systems concern both the dimension that can be checked (for example a thickness of less than 100 micron cannot be checked) and the achievable resolution (not less than 10 micron). In order to overcome these limits, systems with optical probes can be employed. U.S. Pat. No. 6,437,868 relates to an application of this kind, including a reflection optical system which is axially located inside the element supporting the wafer to be checked. By virtue of the characteristics of the material or materials of which it is made (a semiconductor material, typically silicon), the wafer or layers of which the wafer is made is/are partly transparent to the used infrared light and its/their thickness is checked at the opposite side with respect to the side which is grinded. Frequently, it is necessary or convenient to carry out checking at the same side which is grinded, and optical probes enable to carry out such checking, as shown for example in the Japanese patent application published with No. JP-A-08-216016. For the purpose of improving the reliability of the performed checking, it is generally convenient to keep clean the surface whereon the optical probe acts by means of a cleaning fluid such as air or water.
In this way the operation of the optical probe may be affected, in particular owing to unwanted and uncontrollable alterations of the characteristics of the transmitted and/or received beams that are due to the discontinuities of the transmission element and/or to turbulence phenomena in the cleaning fluid. Moreover, the different media generate a partial absorption of the radiation and thus limit the luminous intensity given back to the probe.
As a consequence, it is necessary to employ higher electrical amplifications, and the probe becomes more exposed to background noises.
In the event cleaning air is employed, this latter must be carefully filtered for avoiding to introduce foreign material which would interfere with the checking and would contaminate the surface, causing a consequent increasing of costs. Moreover, the mechanical effect of the air jet on the piece being machined, in particular on very small silicon thicknesses (until 5-10 micron) is not negligible.